Characteristics of PMMA

The polymers obtained by polymerization of acrylic acid and its esters are collectively referred to as acrylic esters, and the corresponding plastics are collectively referred to as polyacrylic plastics, among which methyl methacrylate is the most widely used. The abbreviation code for polymethyl methacrylate is PMMA, commonly known as organic glass. It is the most excellent and cost-effective variety of synthetic transparent materials to date. 1、 Performance polymethyl methacrylate is a rigid, hard, colorless and transparent material with a density of 1.18-1.19g/CM3. It has a low refractive index of about 1.49, a light transmittance of 92%, and a haze of no more than 2%. It is a high-quality organic transparent material. 1. Mechanical properties: Polymethyl methacrylate has good comprehensive mechanical properties and ranks among the top in general plastics. Its tensile, bending, and compression strengths are higher than polyolefins, as well as polystyrene, polyvinyl chloride, etc. Its impact toughness is poor, but slightly better than polystyrene. The mechanical properties of cast polymethyl methacrylate sheets (such as aviation grade organic glass sheets), including tensile, bending, and compression, are higher and can reach the level of engineering plastics such as polyamide and polycarbonate. Generally speaking, the tensile strength of polymethyl methacrylate can reach 50-77 MPa, and the bending strength can reach 90-130 MPa. The upper limit of these performance data has reached or even exceeded some engineering plastics. Its elongation at break is only 2% -3%, so its mechanical properties are basically hard and brittle plastics, and it has notch sensitivity. It is easy to crack under stress, but the fracture surface is not as sharp and uneven as polystyrene and ordinary inorganic glass. 40 ℃ is a second-order transition temperature, equivalent to the temperature at which the lateral methyl group begins to move. Beyond 40 ℃, the toughness and ductility of the material are improved. Methyl methacrylate has low surface hardness and is prone to abrasion. The strength of polymethyl methacrylate is related to the duration of stress exposure, and decreases with increasing exposure time. After stretching, the mechanical properties of polymethyl methacrylate (oriented organic glass) were significantly improved, and the notch sensitivity was also improved. The heat resistance of polymethyl methacrylate is not high. Although its glass transition temperature reaches 104 ℃, its maximum continuous use temperature varies between 65 ℃ and 95 ℃ depending on the working conditions. The thermal deformation temperature is about 96 ℃ (1.18MPa), and the Vicat softening point is about 113 ℃. The heat resistance can be improved by copolymerizing monomers with acrylic methacrylate or ethylene glycol acrylate diesters. The cold resistance of polymethyl methacrylate is also poor, with a brittleness temperature of about 9.2 ℃. The thermal stability of polymethyl methacrylate is moderate, better than polyvinyl chloride and polyoxymethylene, but not as good as polyolefin and polystyrene. Its thermal decomposition temperature is slightly higher than 270 ℃, and its flow temperature is about 160 ℃, so there is still a wide range of melting processing temperatures. The thermal conductivity and specific heat capacity of polymethyl methacrylate are at a moderate level in plastics, with values of 0.19 W/CM. K and 1464 J/Kg. K2, respectively. Due to the presence of polar methyl ester groups on the main chain side, the electrical properties of polymethyl methacrylate are inferior to non-polar plastics such as polyolefins and polystyrene. The polarity of methyl ester group is not too high, and polymethyl methacrylate still has good dielectric and electrical insulation properties. It is worth noting that polymethyl methacrylate and even the entire acrylic plastic have excellent arc resistance. Under the action of an arc, the surface will not produce carbonized conductive pathways and arc track phenomena. 20 ℃ is a second-order transition temperature, corresponding to the temperature at which the side methyl ester group begins to move. Below 20 ℃, the side methyl ester group is in a frozen state, and the electrical properties of the material will be improved compared to when it is above 20 ℃. 3. Chemical and solvent resistant polymethyl methacrylate can withstand relatively dilute inorganic acids, but concentrated inorganic acids can corrode it. It is resistant to alkalis, but warm sodium hydroxide and potassium hydroxide can corrode it. It is resistant to salts and oils, and is resistant to fatty hydrocarbons. It is insoluble in water, methanol, glycerol, etc., but can absorb alcohol swelling and produce stress cracking. It is not resistant to ketones, chlorinated hydrocarbons, and aromatic hydrocarbons. Its solubility parameter is about 18.8 (J/CM3) 1/2, and it can dissolve in many chlorinated hydrocarbons and aromatic hydrocarbons, such as dichloroethane, trichloroethylene, chloroform, toluene, etc. Ethylene acetate and acetone can also dissolve it. Methyl methacrylate has good resistance to gases such as ozone and sulfur dioxide. 4. Weather resistant polymethyl methacrylate has excellent resistance to atmospheric aging. After 4 years of natural aging testing, its samples showed slight weight changes, tensile strength, light transmittance, yellowing of color, significant decrease in silver resistance, and slight improvement in impact strength. Other physical properties remained almost unchanged. 5. Burning polymethyl methacrylate is easy to burn, with a limited oxygen index of only 17.3. 2、 Processing characteristics of methyl methacrylate (Part 1): 1. Methyl methacrylate contains polar side methyl groups and has obvious hygroscopicity, with a water absorption rate generally ranging from 0.3% to 0.4%. It must be dried before molding under drying conditions of 80 ℃ to 85 ℃ for 4-5 hours. 2. Methyl methacrylate has effective and significant non Newtonian fluid properties within the temperature range of molding and processing, and its melt viscosity decreases significantly with increasing shear rate. The melt viscosity is also sensitive to temperature changes. Therefore, for the molding process of polymethyl methacrylate, increasing the molding pressure and temperature can significantly reduce the viscosity of the melt and achieve better flowability. 3. The temperature at which polymethyl methacrylate begins to flow is about 160 ℃, and the temperature at which it begins to decompose is higher than 270 ℃, with a wide processing temperature range. 4. The viscosity of polymethyl methacrylate melt is high and the cooling rate is fast, which makes the product prone to internal stress. Therefore, strict control of process conditions is required during molding, and post-treatment is also necessary after the product is formed. 5. Polymethyl methacrylate is an amorphous polymer with a small shrinkage rate and variation range, generally around 0.5% -0.8%, which is beneficial for forming plastic parts with high dimensional accuracy. 6. Methyl methacrylate has excellent cutting performance, and its profiles can be easily machined into various required sizes. （2） Processing techniques for polymethyl methacrylate include casting, injection molding, extrusion, thermoforming, and other processes. 1. Casting molding is used to form profiles such as organic glass sheets and rods, that is, to form profiles using bulk polymerization methods. The products after casting need to undergo post-treatment, with the post-treatment conditions being 2 hours of insulation at 60 ℃ and 2 hours of insulation at 120 ℃. Injection molding uses granular materials obtained by suspension polymerization and is formed on ordinary plunger or screw injection molding machines. Table 1 shows the typical process conditions for injection molding of polymethyl methacrylate. Table 1 Injection molding process conditions and process parameters for polymethyl methacrylate: Screw injection molding machine, plunger injection molding machine, barrel temperature: ℃, rear part: 180-200, middle part: 190-230, front part: 180-210, 210-240, nozzle temperature: ℃, front part: 210-240, mold temperature: ℃, 40-80, injection pressure: MPa, 80-120, 80-130, holding pressure: MPa, 40-60, 40-60, screw speed: rp.m-1, 20-30. Injection molded products also require post-treatment to eliminate internal stress. The treatment is carried out in a hot air circulation drying oven at 70-80 ℃, and the treatment time depends on the thickness of the product. Generally, it takes about 4 hours. 3. Extrusion molding of polymethyl methacrylate can also be used to prepare organic glass sheets, rods, pipes, sheets, etc. using particle materials produced by suspension polymerization. However, the profiles prepared in this way, especially the sheets, have lower mechanical properties, heat resistance, and solvent resistance than those formed by casting due to the small molecular weight of the polymer. Its advantage is high production efficiency, especially for pipes and other molds used by casting methods. Difficult to manufacture profiles. Extrusion molding can use single-stage or two-stage exhaust extruders, with a screw length to diameter ratio generally around 2025. Table 2 shows typical process conditions for extrusion molding. Table 2 Process conditions and parameters for extrusion molding of polymethyl methacrylate: Sheet, rod, screw compression ratio, 2. Barrel temperature: 150-180 ° C, rear part: 150-180 ° C, middle part: 170-200 ° C, front part: 170-230 ° C, 170-200 extrusion pressure: MPa 2.8-12.4 0.7-3.4, inlet temperature: 50-80 ° C, mold temperature: 180-200 ° C. Thermoforming is the process of making various sizes and shapes of organic glass sheets or sheets into products. The raw materials are cut into the required size and clamped on the mold frame, heated to soften them, and then pressed tightly against the mold surface to obtain the same shape as the mold surface. After cooling and shaping, the edges are trimmed to obtain the product. Pressurization can be achieved through vacuum drawing or by directly applying pressure to a convex mold with a profile. The hot forming temperature can refer to the recommended temperature range in Table 3. When using rapid vacuum low draft forming products, it is advisable to use temperatures close to the lower limit. For deep draft products with complex shapes, temperatures close to the upper limit should be used, and normal temperatures are generally used. Table 3: Lower Limit Temperature, Upper Limit Temperature, Normal Temperature, Cooling Temperature: 149 ℃, 193 ℃, 177 ℃, 85 ℃. In addition, profiles can also be machined using methods such as turning, milling, drilling, and cutting. 3、 The application of polymethyl methacrylate is widely used as a high-performance transparent material in the following areas: 1. Lamps and lighting fixtures, such as various household lamps, fluorescent lampshades, car taillights, signal lights, and road signs. 2. Optical glass, such as manufacturing various lenses, mirrors, prisms, TV screens, Fresnel lenses, and camera transparency zeros. 3. Prepare various instrument dials, covers, and scales. 4. Prepare optical fibers. 5. Product advertising display windows and billboards. 6. Aircraft cockpit glass, bulletproof glass for airplanes and cars (with intermediate interlayer material). 7. Various types of medical, military, and architectural glass. 4、 Oriented organic glass polymethyl methacrylate sheet can be obtained by oriented stretching above the glass transition temperature and cooling in the stretched state, resulting in a sheet with molecular chains in an oriented state, which is called oriented organic glass. The performance of oriented organic glass is significantly improved compared to non oriented organic glass. （1） The directional stretching method heats high-quality organic glass sheets to 105-110 ℃ (slightly higher than Tg), quickly places them in stretching equipment equipped with fixed fixtures and water cooling devices, stretches them to the required stretching degree, stops stretching and maintains cooling under tension. For circular glass panels, they are uniformly stretched radially in multiple directions; For square glass panels, they are stretched in two perpendicular directions. After stretching, the molecular chains of the organic glass sheet undergo biaxial orientation along the plane of the sheet and are frozen. （2） Compared with unstretched organic glass sheets, oriented organic glass molecular chains exhibit improved tensile strength, bending strength, resistance to silver streaks, crack propagation, modulus, elongation at break, and impact strength due to their ordered orientation. The improvement of the mechanical properties mentioned above is related to the degree of tension. As the degree of tension increases, the degree of performance improvement also increases. However, when the degree of tension exceeds 50% -60%, except for the continued improvement in impact strength, other properties remain basically unchanged. Therefore, the tensile strength should generally be controlled at around 60%, at which point the material has good overall performance.